A Possible Soft-shelled Eldredgeops from Ohio

The actual past is brittle . . . . —David Mitchell

Eldredgeops rana milleri, Silica Shale, Middle Devonian, Paulding, Ohio.
Eldredgeops milleri, Silica Shale, Middle Devonian Epoch, Paulding, Ohio. The glabella has been pushed in, without cracking. Specimen is about 2.4 cm across the genals.

A singular, commercially-collected Eldredgeops specimen in the collection shows an unusual style of preservation. Rather than having been rigid and brittle as one would expect for an invertebrate exoskeleton composed of calcite, this specimen possessed a fair degree of flexibility, and, as a result, was deformed after burial.

Eldredgeops rana milleri, Silica Shale, Middle Devonian, Paulding, Ohio
Eldredgeops milleri, Silica Shale, Middle Devonian Epoch, Paulding, Ohio. This specimen appears to have been squashed, presumably upon burial and compaction. Specimen about 2.4 cm across the genals.

Specifically, this trilobite appears to have been sheared or “smeared out” laterally. Very little cracking has occurred, however. A few major cracks exist in the axial and pleural lobes. Only one major crack appears in the cephalon. Overall, the degree of cracking is not commensurate with the level of deformation, suggesting a degree of softness. Cracking in a few areas only may have indicated a lack of uniform mineralization and brittleness.

Drage and Daley (2016) reviewed much of the literature surrounding “soft-shelled” trilobite preservation. Some past authors have interpreted paleness, thinness, wrinkling, and flattening of the shells of specimens to be the result of the burial of recently molted, and therefore not completely mineralized individuals—especially if these specimens occur in the context of other specimens exhibiting typical preservational patterns.

Although this specimen is odd and suggestive of “soft-shelled” preservation, because it exists out of context of other fossils from its stratum much doubt exists about its origin. Perhaps unusual geochemistry or diagenetic forces were at work here. Detailed taphonomic observations in the field could have helped sort this one out.

Eldredgeops rana crassituberculata, Silica Shale, Middle Devonian, Sylvania, Ohio
Eldredgeops crassituberculata, Silica Shale, Middle Devonian Epoch, Sylvania, Ohio. This specimen shows the typical style of preservation for Ohio “Phacops.” Matrix fills tiny cracks that likely formed during compaction of the brittle exoskeleton. Specimen about 4.6 cm long.

Reference

Drage, H. B., and Daley, A. C. 2016. Recognizing moulting behavior in trilobites by examining morphology, development, and preservation: Comment on Blazejowski et al. 2015. Bioessays 38 (10): 981-990.

©2016 Christopher R. Cunningham. All rights reserved. No text or images may  be duplicated or distributed without permission.